Dry etching is a known technique for etching an etching object made of a silicon compound such as silicon, silicon dioxide, or silicon nitride, a metal such as aluminum, tungsten, molybdenum, or titanium, or a polymer such as a resist, by use of a plasma etching apparatus, a reactive sputter etching apparatus, or the like. This technique includes various types, such as reactive ion etching (RIE), electron cyclotron resonance (ECR) etching, and downflow etching. Among these types, having characteristics of mass productivity and anisotropic etching which enables fine pattern formation, RIE, RF-bias ECR etching, and the like have conventionally been in wide use in manufacture of semiconductor devices.
In a conventional RIE apparatus, first, a wafer is placed on a radio frequency (RF) electrode installed in a chamber, and the chamber is degassed. A plasma generating gas is then introduced into the chamber, and the chamber is controlled to have a predetermined internal pressure through adjustment of the flow and the exhaust velocity of the gas. Next, a predetermined radio frequency wave is applied to the RF electrode through an RF matching box to generate plasma in the chamber. Then, the wafer is etched by exposing a surface of the wafer to the plasma to react therewith. In this event, a required etching mask (resist) is applied to the surface of the wafer so that only object portions on the wafer are selectively etched. At the time of the etching, the wafer is heated by heat generated from chemical reaction with the plasma and by incident energy of collision of ions in the plasma state, or the like. Since the heat burns the resist on the wafer, the wafer needs to be cooled. Further, since the etching process is easily affected by the temperature, precise control of the wafer temperature is important in fine pattern formation.
For such temperature control, it is necessary to appropriately adjust the temperature of the RF electrode on which the wafer is placed, by using a medium such as cooling water, and also to bring the wafer and the RF electrode into tight contact to each other to increase the thermal conductivity therebetween. For those reasons, conventionally, for efficient temperature control of the wafer, an electrostatic chuck (substrate) is provided on the RF electrode, and the wafer is brought into tight contact with the electrostatic chuck so that the contact area may be increased for a thermal contact.
In recent wafer processes, since fine temperature control during the processes is very significant, using an electrostatic chuck equipped with a heater has become mainstream. Such a heater-equipped electrostatic chuck generally has a structure in which a film heater is sandwiched between a metallic base (a base plate which is made of aluminum or the like and inside which a cooling flow channel is provided to pass a cooling medium such as water) and an electrostatic chuck substrate which is made of a ceramic material or the like and in which an electrode layer for electrostatic attraction is embedded. This heater is bonded to the base plate with an adhesive layer interposing therebetween.
The adhesive layer interposing between the heater and the base plate (also referred to as a “heater adhesive layer” below for convenience) needs to be thick enough to even out and stabilize the surface temperature (about 0.5 to 2 mm). When the heater adhesive layer has voids inside, thermal conductivity becomes uneven to cause variation in the temperature distribution in the surface. For this reason, it is important for the adhesive layer not to have voids inside after the heater is bonded to the base plate. Further, since the variation in the surface thickness of the adhesive layer also affects the temperature distribution, the thickness variation needs to be reduced as much as possible.
The conditions for recent wafer processes have become severe. Since the temperature used is high, the heater has high power. To enhance responsiveness and throughput of the temperature control, an improvement in cooling efficiency is necessary. Accordingly, the heater adhesive layer requires a material having high thermal conductivity.
As a technique related to such a conventional art, there is known an apparatus in which a metal plate and a heater are bonded to a top surface of a temperature-controlled mount with a first layer made of an adhesive material interposing therebetween, and a layer made of a dielectric material is then bonded to a top surface of the metal plate with a second layer made of an adhesive material interposing therebetween (for example, Japanese National Publication of International Patent Application No. 2008-527694).
In addition, there is known a technique in which damper layers are provided between a lower insulating layer and a base member of an electrostatic chuck. The damper layers are made of composite materials across which a ratio between a matrix metal and an additive changes such that a thermal expansion rate gradually increases from the lower insulating layer side to the base member side within a predetermined range of the thermal expansion rate of the lower insulating layer and the thermal expansion rate of the base member (for example, Japanese Laid-open Patent Publication No. 10-41377).
Moreover, there is known a technique for a wafer supporting member which includes: a holding part in which one of main surfaces of a plate body serves as a mount surface to mount a wafer; a heater part in which an insulating resin has heaters embedded thereinside and has recessed portions in a surface thereof, and in which a resin having a different composition from the insulating resin is filled to embed the recessed portions; and a conductive base part. The heater part is sandwiched between the holding part and the conductive base part (for example, Japanese Laid-open Patent Publication No. 2005-277074).
As described above, in the conventional heater-equipped electrostatic chuck, the thermal conductivity of the heater adhesive layer interposing between the heater and the base plate needs to be increased, and to that end, an adhesive having high thermal conductivity (typically, a silicone-based resin) has to be used.
However, an adhesive having high thermal conductivity generally has high filler-content percentage and therefore has high viscosity (in the case of a silicone resin, more than 500 P (pores)), and this decreases workability. Specifically, since application of a high-viscosity material with an even thickness is difficult, a problem arises that the work from the application to attachment of the heater to the base plate cannot be carried out smoothly.
In addition, since high viscosity causes poor defoaming performance, there is a problem that voids tend to remain in the heater adhesive layer after cure (after the heater is bonded to the base plate). Further, having a high filler-content percentage and high viscosity, the adhesive increases in hardness after the cure; consequently, stress generated due to thermal stress repeatedly produced while the electrostatic chuck is used cannot be relaxed sufficiently, causing a problem that the heater is peeled off from the adhesive interface (a decrease in the reliability as an electrostatic chuck).
Regarding a method for bonding the heater part to the conductive base part to improve the temperature distribution of a completed electrostatic chuck, JPP No. 2005-277074 given above discloses that the adhesive layer is stacked multiple times, and the heater part and the base part are joined to each other in a reduced pressure atmosphere. However, JP-A 2005-277074 does not particularly mention about using adhesives different in property from each other for the adhesive layer stacked on the heater part and for the adhesive layer for joining the heater part to the base part.